1. Field of the Invention
This invention relates to a new and useful invention concerning a system and a method for mounting an article, such as an impeller and/or a gear mechanism, onto a drive shaft. In particular, this invention relates to a system and method for mounting one or more impellers on a drive shaft for use, for example, in a centrifugal compressor.
2. Description of the Prior Art
The usual method for mounting articles on drive shafts is to employ keys and keyways. Keyways are machined into the drive shaft and into the central bore of the article, and keys are provided to connect the article to the drive shaft with a view to providing a secure fit sufficient to transmit torque. When conventional keys and keyways are used to mount an article such as an impeller on a drive shaft to transmit torque, positive torque is transmitted but the shaft is weakened because of high stress raised by the keyway. The mounted article is also weakened by the keyway in the bore. It is therefore desirable not to employ keys and keyways to transmit torque to mounted articles, particularly impellers.
Accordingly, it is an object of the present invention to provide a system for mounting an article on a drive shaft without employing keys and keyways.
It is yet another object of the present invention to provide a method of mounting an article on a drive shaft without employing keys and keyways.
These and other objects are accomplished by the present invention in a system comprising tapered threads on an impeller shaft; a threaded collar having threads effective to engage the tapered threads on the impeller shaft; at least one impeller on the impeller shaft in contact with the threaded collar; a clamping collar adjacent to the impeller, on a side of the impeller opposite the threaded collar; and an arrangement applying a generally axial force against the clamping collar in a direction toward the threaded collar so that the threaded collar slides up on the tapered threads. In a preferred embodiment, the force applying arrangement comprises a clamping collar having an annular formation extending axially into contact with the impeller only in an area of the impeller spaced radially from its central bore, and a threaded arrangement with the impeller shaft for applying an axial force to the clamping collar. Because the axial force is applied only in an area spaced radially from the central bore of the impeller, the force deforms the impeller such that the central bore is slightly reduced at the side of the impeller opposite the force applying arrangement. As a result, the radial tolerances between the impeller shaft and that side of the impeller are reduced, thereby better centering the impeller on the shaft and decreasing any tendency of the impeller to vibrate, particularly at the high speeds of a centrifugal compressor, for example, 17,000 RPM. In some cases, the deformation results in radial clamping of the impeller on the impeller shaft, thereby enabling torque to be transmitted to the impeller through the area of clamping.
In one embodiment, first and second opposing impellers are positioned adjacent to one another, each having a central bore. The first impeller is positioned adjacent the clamping collar and comprises an annular formation extending axially into contact with the second impeller in an area of the second impeller spaced radially from the central bore of the second impeller so that the first impeller contacts the second impeller only along the annular rim. As a result, the second impeller exhibits a deformation, vibration reduction and, in some cases, radial clamping and torque transmission, as was just described in connection with the first impeller.
The threaded collar slides up and tightens on the tapered threads as the axial force is applied, whereby the threaded collar is held tightly on the impeller shaft, and torque is transmitted through the threaded connection from the impeller shaft to the threaded collar. Torque is transmitted from the threaded collar to the impeller through their mutually contacting surfaces. Where there are two impellers, torque is transmitted from the impeller adjacent the threaded collar to the other impeller through their mutually contacting surfaces.
The slight deflection of the clamping collar due to its shape, and the slight deflection of the impellers by having the contact areas spaced radially outward creates a spring effect so that even though the impellers may change in temperature with respect to the shaft, and try to expand, there is enough spring action and allowable deflection in the impellers and the collar that the assembly will remain tight under all operating conditions.
Other advantages are that:
1. The impellers are easy to machine and have no stress raisers with keys in the shaft. Therefore, this gives them additional strength.
2. The shaft itself is smooth and round and has no stress raisers in it, and therefore the minimum possible shaft diameter can be used.
3. The shaft diameter being small can allow a ball bearing supporting it to be slid over the shaft, and this permits the bearing supporting the impellers to be as close as possible to the impellers, which are unsupported on the side opposite to the ball bearing. This assures minimum vibration during high-speed operation, and assures that the natural frequency or critical speed is well above operating speed.
4. The shaft between the bearing just mentioned and a bearing distal to the impellers can be much larger and therefore stiffer so that minimum vibration is assured by being able to have the shaft within the impellers be as small as possible and being able to slide the closer bearing over the shaft itself. This makes for a strong assembly that provides maximum strength against vibration, and also permits the impellers to utilize their full strength without having any stress raisers such as keyways in the bore of the impeller. This invention permits high strength, high speed, mounting of the impellers so that the optimum operating conditions can be achieved.